Biochemical differentiation, leading in the course of normal development to enzyme patterns specific for particular tissues, includes the formation of tissue-specific enzymes, the quantitative redistribution of "ubiquitous" enzymes and the emergence of isozymes characteristic of adult tissues. Identification of physiological stimuli responsible for some aspects of the enzymic maturation process will help to elucidate both normal development and some of the possible causes for abnormal development. Studies will include enzyme measurements over a period of several weeks in tissues of rats injected with hydrocortisol or thyroxine in utero or immediately after birth to assess the hormone effects on the developing enzyme pattern instead of only on single responsive enzymic reactions.